1. Field of the Invention
The present invention relates to an electronic circuit device in which the gold-plated terminal of an electronic part is connected with a solder to a circuit substrate, a method of connecting the gold-plated terminal to the circuit substrate with a solder, and the solder for use in connecting the gold-plated terminals.
2. Description of Related Art
Hitherto there has been proposed such an example of electronic circuit device as mentioned below, in order to raise the calculation speed and operating efficiency of an electronic computer, particularly a supercomputer.
A plurality of large scale integrated semiconductor devices, i.e., LSI chips are mounted by connecting with minute solder balls. This solder-ball connecting method is disclosed in, for example, IBM Journal, May 1969, pp. 251-265.
Since a considerable amount of electric power is supplied to the LSI chips in the actual working of the computer, the problem of heat occurs. That is, the overheating of the LSI chips breaks the chips. In order to prevent this overheating, a cooling cap is arranged over the LSI chips on the substrate. In order to connect the substrate to an outside circuit in an electric and mechanical manner, connecting pins are provided on the substrate on the back side thereof. These connections require high reliability in a mechanical, electrical and chemical point of view. As a solder for connecting have been widely used Sn-Pb alloys, particularly an eutectic solder consisting of 63.0 wt % of Sn and 37.0 wt % of Pb (hereinafter referred to as Sn37Pb solder). This solder is most generally used since it has a good elongation property and a relatively low and constant melting point of 183.degree. C. so that it is easily operable.
Circuit substrates having LSI or IC mounted thereon are normally made of non-metal materials such as ceramics or resins. Therefore, in order to connect these materials with a solder, the areas of the substrate to be connected are often plated with a metal to secure the wettability of the solder to the substrate. In the case of connecting with minute solder balls, round-shaped minute metal platings are normally provided on the substrate. Particularly, gold platings are preferred as the metal platings, because oxides are hardly formed on the gold platings and the gold platings are well reactive with the solder.
However, the gold-plated base materials (i.e., connecting terminals) produce a gold-tin compound, as mentioned below, when connected with the Sn37Pb solder. In this case there is a problem that coarsening of the compound causes connected portions to be embrittled so that the reliability of connection is lowered. This problem is disclosed in, for example, Proc. Tech. Programme Intern., 1968, pp. 211-231.
In order to raise the calculation and operating efficiency of the supercomputer, various packaging methods have been proposed and the structure of the electronic circuit device is multilayered and more complicated. For example, in the abovementioned electronic circuit device, an LSI chip is mounted on each of carrier chips and sealed with a cap. A plurality of the thus cap-sealed carrier chips are mounted on a circuit substrate having a larger surface area than that of the carrier chip. When the electronic circuit device having such a structure is to be made, the LSI chip is firstly connected with minute solder balls, and then the cap and the carrier chip are joined together. The melting point of a solder to be used in this joining must be lower than that of a solder used in the connection of the LSI chip in order to allow the connected portions not to be molten. The same things are applicable to the subsequent steps. A lower melting point solder is required by a later step. Thus, the electronic circuit device having a multilayered and more complicated structure requires solders having different melting points corresponding to the connecting temperature required. For this purpose, Pb5Sn, Pb10Sn, Pb37Sn, Pb37Sn18Bi and the like have been generally used. In addition to these Sn-Pb system alloy solders, Sn-Ag system solders containing a small amount of silver are also known. However, these Sn-Ag system solders have not been studied, because they are not operable to connection of fine patterns in which LSI chips are connected to a circuit substrate with minute soldering balls. The first reason is that since the solder contains a large amount of tin there is such a danger that thin whiskers and low-temperature tin-transformed phases are formed. The production of the tin whiskers introduces shortage of fine electrodes to adjacent ones with high probability. Furthermore, the low-temperature tin-transformed phases is produced by lattice transformation from the .beta.-phase to the .alpha.-phase, thereby causing the expansion of volume to break the connected portions. The second reason is that the presence of silver easily causes migration. The Sn-Ag system solders are disclosed in, for example, Japanese Patent KOKAI (Laid-Open) No. 62-166091. However, this solder disclosed in the KOKAI is in a cream state. Therefore, this solder cannot be used for the connection requiring to use the minute soldering balls.
Gold-plated base materials are well wettable to the Sn37Pb solder at the plated areas. This is because tin in the Sn37Pb solder in a molten state is easily reactive with gold. Thus, the solder is spread on the plated areas while forming a gold-tin compound comprises principally of AuSn.sub.4. The gold-tin compound is very hard and brittle. Furthermore, if the gold content of the solder-connected portions is high, the resultant gold-tin compound has a coarsened crystal grains. If the connected portions have larger gold-tin crystal grains, force externally added may be concentrated onto the boundary between the compound and the base material to cause the connected portions to break since the portions have gotten hard and brittle. Thus, the solderconnected portions have conspicuously lowered reliability. This phenomenon can be quantitatively rated by determining the tensile elongation of the solder-connected portions. A relationship between the gold content of the Sn37Pb solder-connected portion and the tensile elongation is shown in FIG. 2. Furthermore, FIGS. 3(a), 3(b), 3(c) and 3(d) are electron microphotographs showing the microstructures of metal of the connected portions. That is, as seen from FIG. 2, the tensile elongation (expressed in terms of tensile fracture elongation) is conspicuously lowered at the gold content of 3 wt %. Accordingly, as seen from FIGS. 3(a) to 3(d), the crystal grains of the gold-tin compound are coarsened as shown with an arrow mark.
The tensile testing method and the method for the observation of the microstructure used in the determination above will be detailedly discussed later. From these figures, it is seen that if the gold content of the Sn37Pb solder exceeds 3 wt %, the tensile elongated is 20% or less and the connected portion becomes hard and brittle. In such a state, force externally added is concentrated onto the boundary of the gold-tin compound to cause the connected portion to be broken. One of ways for preventing this breaking is considered to make the thickness of the gold plating as small as possible, or to increase an amount of the solder so as to adjust it to 3.0 wt % or less. This way is not practical for the following reasons.
In plating a predetermined area on a modular circuit substrate made of ceramics in an electronic circuit device, with gold, the area is often firstly plated with nickel or cobalt having a good affinity to the ceramics and then with gold on the nickel or cobalt. After the plating, in order to improve the adhesion between these two kinds of the platings, they are generally heat treated and held at a temperature of several hundreds. In this case, mutual diffusion takes place between the two kinds of the plating. If the gold plating is thin, then the underlayer, nickel or cobalt is exposed on the surface of the gold plating, which causes the wettability to be deteriorated. For the purpose of avoiding this, the gold plating is required to have a thickness of at least several microns.
In electronic circuit devices, one modular substrate has various parts mounted thereon. The solder amount in the connected portions varies depending upon the kind of parts to be connected. The substrate is plated at a time for technical and economical reasons and the thickness of the plating is uniform. Thus, the thickness of the plating cannot be changed according to the kind of the parts to be connected in one substrate. Therefore, the gold contents of the solder cannot be controlled at a time for every parts using the different solder amounts. In the case where a small size part is connected with a solder in a small amount, the gold content of the solder is high at the connected portion. To avoid this, the solder amount must be increased, so the part must be made larger. This undesirably inhibits the high-densification of the parts mounted on the substrate.
As is seen from the above, the conventional method of connecting parts using a solder of Sn37Pb have such problems that connected portions have reliability reduced due to the presence of gold and the solder amount is increased to avoid the reduction of reliability, so that the high-densification of parts on a substrate is prevented.
The above-mentioned literatures and Japanese Patent KOKAI (Laid-Open) No. 62-166091 disclose a special solder in a cream state (Sn-Ag solders containing a flux) other than the Sn-Pb alloys. However, such a solder has never been studied for use in fine connections requiring high reliability in the field of electronic circuit devices, for the reasons as mentioned above.